Final answer:
The constant ground auxiliary lead in a Power Probe is used to ensure a reliable ground connection, providing safety by keeping the appliance at zero volts relative to the earth and allowing for protective measures like the tripping of circuit breakers.
Step-by-step explanation:
A Power Probe, which comes with a constant ground auxiliary lead, is used to maintain a safe and reliable ground connection within an electrical system. The constant ground serves as a failsafe. In the case of electrical appliances, the grounding of metal cases ensures they are at zero volts relative to the earth, effectively making them safe to touch and preventing electrical shock from insulation failures. It also allows for protective devices like circuit breakers to trigger and prevent damage or injury. For appliances without conducting cases, known as doubly insulated, a two-prong plug can suffice as they possess impact-resistant plastic cases and present less risk of electric shock.